This invention relates to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. The molten metal forms shells on the casting surfaces that join and pass through the nip between the casting rolls as thin metal strip is cast downwardly from the nip.
The casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the casting rolls so as to constrain the two ends of the casting pool against outflow. Side dams at the ends of the casting rolls prevent leakage of molten metal from the casting pool and maintain the casting pool at a desired depth. As the casting rolls are rotated, the side dams experience frictional wear, causing arc-shaped grooves to form in the side dams along the circumferential surfaces of the casting rolls. In order to compensate for this wear, the side dams are movable to gradually shift inward under compression forces in order to maintain the seal with the casting rolls.
The useful life of the side dam has traditionally been limited by the depth of the arc-shaped grooves that can be made without risk of solidified sculls forming and dropping through the nip between the casting rolls and forming defects, called “snake eggs,” in the cast strip. It has been proposed to increase the life of the side dams by making them vertically moveable so they can be moved upward. That way multiple arc-shaped grooves can be worn into the same side dam, thereby increasing the useful life of the side dam. Examples of these past proposals for increasing the useful life of side dams are described in U.S. Pat. No. 7,066,238 and U.S. Patent Publications Nos. 2006/0054298 and 2010/0101752. However, there continues to be a need for a way to improve the operational life of side dams.
In any event, the arc-shaped grooves tend to promote the formation of solidified sculls in the molten metal that tend to cause the formation of snake-egg defects in the cast strip. Where the side dams engage with the ends of the casting rolls, the amount of cooling of the metal shells on the casting rolls is higher than in the center of the rolls. The solidified sculls can form in solidified shells adjacent the side dams in the arc-shaped grooves and may give rise to ‘snake egg’ defects in the formed metal strip.’ Such snake eggs can cause not only defects in the cast strip but may also cause the continuous metal strip to break or otherwise rupture as the strip is formed. Accordingly, there remains a need for a twin roll caster and method of operating the same, that reduces the likelihood of formation of snake eggs by inhibiting the formation of arc-shaped grooves in the side dams adjacent the casting rolls, while extending the operating life of the side dams.
Disclosed herein is a twin roll caster comprising:
a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a nip there between through which thin cast strip can be cast, and supporting a casting pool of molten metal on the casting surfaces above the nip;
a pair of side dams positioned to engage end portions of the casting rolls adjacent the nip to laterally confine said casting pool; and
a side dam support applying a compression force against at least one of said side dams at an upward angle between 15° and 45° relative to an axis of said casting rolls.
A side dam support may be provided at each end portion of the casting rolls applying an angular compression force against each side dams at an upward angle between 15° and 45° relative to an axis of said casting rolls. In any case, during operation said side dam is worn by said end surfaces of said casting rolls to form a slantwise groove in each side dam. The slantwise groove may be in the form of V-shaped arcuate grooves.
The side dam supports may comprise a lateral pushing apparatus to push said side dam against said end surfaces and a vertical pushing apparatus to adjust the height of said lateral pushing apparatus, and the lateral pushing apparatus and the vertical pushing apparatus may be adapted to operate at the same time.
In addition, a control device may be adapted to control said vertical pushing device and said lateral pushing device to provide a target compression angle. Alternatively, the side dam support comprises a slantwise pushing apparatus adapted to push said side dam against said end surfaces of the casting roll at a target compression angle. The compression angle may be dynamically controlled.
Also disclosed is a method of controlling a twin roll caster having two laterally positioned casting rolls forming a nip there between and two side dams positioned adjacent opposite end portions of the casting rolls to enable a casting pool to be formed on the casting rolls above the nip, the method comprising the steps of:
providing a compression device to apply a compression force against said side dams inwardly towards end portions of said casting rolls at an upward angle, and
forming slantwise grooves worn in the side dams by said end portions of said casting rolls.
The method of controlling a twin roll caster may include causing the compression device to apply the compression force to form slantwise grooves in a series of V-shaped grooves by the steps of determining a target step thickness and spread angle for V-shaped grooves, and control the compression device to provide the compression angle to provide said target step thickness and spread angle in the side dams. The method may include providing a lateral force parallel to an axis of said casting rolls and a vertical force perpendicular to said lateral force to form a resultant compression force at said compression angle.
The method may include providing a control device to determine said compression angle and communicate with said compression device to adjust said compression angle. The compression device may be a slantwise compression device to provide said compression force at said compression angle. Said slantwise compression device may also include an angular adjustment member for adjusting said compression angle.
Also, said slantwise compression device may include a displacement measuring device. The further steps of communicating a displacement value from said displacement measuring device to a control device, determining a target compression angle based on said displacement value and a target spread angle, and communicating a value to said angular adjustment member to adjust said slantwise compression device to said target compression angle. Said spread angle may be either variable or fixed